Ligand-Dependent Selective Formation of Unique Silylpalladium

Sep 9, 2010 - On the other hand, 6b afforded a mononuclear palladium complex with a ... Wei Huang. Chinese Journal of Chemistry 2017 35 (4), 507-511 ...
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Organometallics 2010, 29, 4406–4409 DOI: 10.1021/om100626k

Ligand-Dependent Selective Formation of Unique Silylpalladium Complexes by the Reaction of 1-(Dimethylsilyl)-2-silylbenzene and [{1,2-C6H4(SiMe2)(SiH2)}Pd(R2PCH2CH2PR2)] Yong-Hua Li† and Shigeru Shimada* National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. †Present address: College of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, People’s Republic of China. Received June 30, 2010 Summary: The reaction of 1-(dimethylsilyl)-2-silylbenzene (4) and [{1,2-C6H4(SiMe2)(SiH2)}Pd(R2PCH2CH2PR2)] (6a, R = Me; 6b, R = Et) selectively afforded unique palladium complexes, depending on the phosphine ligands. For 6a, a dinuclear palladium complex with five Si-Si bonds was obtained, in which two 6a molecules were bridged by a cyclic dimer of 4 at the SiH2 silicon atom of 6a. On the other hand, 6b afforded a mononuclear palladium complex with a benzotrisilacyclopentene moiety, which was formed by two Si-Si coupling reactions between the SiH2 group of 6b and the SiH3 and SiMe2H groups of 4.

Transition-metal complexes are useful catalysts for the transformation of silicon compounds in a number of important reactions such as hydrosilylation reactions of unsaturated organic compounds,1 dehydrogenative Si-Si coupling reactions,2 and bis-silylation reactions.3 An understanding of the basic reactivity between silicon compounds and transition-metal complexes is important to clarify the reaction mechanism of catalysis as well as to develop more efficient catalysts and new catalytic reactions. Group 10 transition-metal complexes are the most important catalysts for such reactions, and their reactivity toward hydrosilanes have attracted increasing interest. Recent studies resulted in a number of unique structures and interesting reactivities.4,5 We have been studying the catalytic transformation of 1,2-disilylbenzenes as well as stoichiometric reactions of (1) (a) Marciniec, B.; Maciejewski, H.; Pietraszuk, C.; Pawluc, P. Hydrosilylation: A Comprehensive Review on Recent Advances; Marciniec, B., Ed.; Springer: Berlin, 2009; Advances in Silicon Science, Vol. 1. (b) Marciniec, B.; Gulinski, J.; Urbaniak, W.; Kornetka, Z. W. Comprehensive Handbook on Hydrosilylation; Marciniec, B., Ed.; Pergamon Press: Oxford, U.K., 1992. (2) Corey, J. Y.; In Advances in Silicon Chemistry. A Research Annual; Larson, G. L., Ed.; JAI Press: Greenwich, CT, 1991; p 327. Gauvin, F.; Harrod, J. F.; Woo, H. G. Adv. Organomet. Chem. 1998, 42, 363. (3) Suginome, M.; Ito, Y. Chem. Rev. 2000, 100, 3221. (4) Corey, J. Y.; Braddock-Wilking, J. Chem. Rev. 1999, 99, 175. (5) Recent examples: (a) Yamada, T.; Mawatari, A.; Tanabe, M.; Osakada, K.; Tanase, T. Angew. Chem., Int. Ed. 2009, 48, 568. (b) Arii, H.; Takahashi, M.; Nanjo, M.; Mochida, K. Organometallics 2009, 28, 4629. (c) McBee, J. L.; Tilley, T. D. Organometallics 2009, 28, 3947. (d) BerthonGelloz, G.; de Bruin, B.; Tinant, B.; Marko, I. E. Angew. Chem., Int. Ed. 2009, 48, 3161. (e) Adhikari, D.; Pink, M.; Mindiola, D. J. Organometallics 2009, 28, 2072. (f) West, N. M.; White, P. S.; Templeton, J. L.; Nixon, J. F. Organometallics 2009, 28, 1425. (g) Hanada, S.; Tsutsumi, E.; Motoyama, Y.; Nagashima, H. J. Am. Chem. Soc. 2009, 131, 15032. (h) Tanabe, M.; Ito, D.; Osakada, K. Organometallics 2008, 27, 2258. (i) Arii, H.; Takahashi, M.; Noda, A.; Nanjo, M.; Mochida, K. Organometallics 2008, 27, 1929. (j) Braddock-Wilking, J.; Corey, J. Y.; French, L. M.; Choi, E.; Speedie, V. J.; Rutherford, M. F.; Yao, S.; Xu, H.; Rath, N. P. Organometallics 2006, 25, 3974. (k) Iluc, V. M.; Hillhouse, G. L. Tetrahedron 2006, 62, 7577. pubs.acs.org/Organometallics

Published on Web 09/09/2010

1,2-disilylbenzenes with group 10 transition-metal complexes.6 The latter resulted in the discovery of a diverse range of unique structures depending on the metals, the structures of 1,2-disilylbenzenes, and the ligands. In a previous report, we showed that the reaction of 1,2-disilylbenzene (1) with Me2Pd(dmpe) (dmpe=1,2-bis(dimethylphosphino)ethane) afforded the tetrakis(silyl)palladium(IV) complex 3 through the intermediacy of the bis(silyl)palladium(II) complex 2 (Scheme 1).7 Here, we report that changing 1 to 1-(dimethylsilyl)-2-silylbenzene (4) resulted in two very different structures, 7 and 8, which were selectively formed by the reaction of 4 with [{1,2-C6H4(SiMe2)(SiH2)}Pd(dmpe)] (6a) or [{1,2C6H4(SiMe2)(SiH2)}Pd(depe)] (6b,; depe = 1,2-bis(diethyl)phosphinoethane), depending on the steric bulkiness of the phosphine ligands.8

Results and Discussion Recently, we reported that a self-condensation reaction of the bis(silyl)palladium(II) complex 6a took place at 80 °C or higher to give unique complexes, including tri- and tetranuclear palladium complexes that possess a palladium center bonded to five silicon atoms.9 In order to examine the effect of phosphine ligand bulkiness on the reaction, we had interest in the reactivity of a similar bis(silyl)palladium(II) complex, 6b, bearing a bulkier depe ligand. In the case of the corresponding bis(silyl)platinum(II) complexes, both [{1,2-C6H4(SiMe2)(SiH2)}Pt(dmpe)] and [{1,2-C6H4(SiMe2)(SiH2)}Pt(depe)] showed a similar reactivity and easily formed their dimeric platinum(IV) complexes [(dmpe)PtIV(H)({1,2-C6H4(SiMe2)(μ-SiH)}]2 and [(depe)PtIV(H)({1,2-C6H4(SiMe2)(μ-SiH)}]2, respectively.10 The bis(silyl)palladium complex 6b was obtained from 4 and palladium(0) phosphine complex 5b by a procedure similar to that for 6a (Scheme 2),9 and its structure was confirmed by NMR spectroscopy and X-ray diffraction. As expected, complex 6b is more thermally stable than 6a, and it was proved to be too stable for self-condensation; no reaction took place even after heating at 165 °C (oil bath temperature) in toluene in a sealed tube. (6) (a) Shimada, S.; Tanaka, M. Coord. Chem. Rev. 2006, 250, 991. (b) Shimada, S.; Uchimaru, Y.; Tanaka, M. Chem. Lett. 1995, 223. (7) Shimada, S.; Tanaka, M.; Shiro, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 1856. (8) Some results were briefly described in a review article.6a (9) Shimada, S.; Li, Y. H.; Choe, Y. K.; Tanaka, M.; Bao, M.; Uchimaru, T. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 7758. (10) Shimada, S.; Rao, M. L. N.; Li, Y. H.; Tanaka, M. Organometallics 2005, 24, 6029. r 2010 American Chemical Society

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Scheme 1. Reaction of 1,2-Disilylbenzene with Me2Pd(dmpe)

Scheme 2. Formation of Complexes 7 and 8

Figure 1. Thermal ellipsoid plot (50% probability level for Pd, P, Si, and C atoms) of compound 7. Hydrogen atoms bonded to carbon atoms are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Pd(1)-P(1), 2.3151(9); Pd(1)-P(2), 2.3538(7); Pd(1)-Si(1), 2.3806(7); Pd(1)-Si(2), 2.3537(9); Si(2)-Si(3), 2.3594(11); Si(2)-Si(4), 2.3337(12); P(1)-Pd(1)-P(2), 85.91(3); P(1)-Pd(1)-Si(1), 95.56(3); P(2)-Pd(1)-Si(2), 99.50(2); Si(1)Pd(1)-Si(2), 79.04(2); Pd(1)-Si(2)-Si(3), 113.27(3); Pd(1)Si(2)-Si(4), 122.11(3); Si(3)-Si(2)-Si(4), 85.61(4).

Then we examined the reaction of 6b with another molecule of 4 and found it did proceed at 90 °C to give the new complex 7 selectively (71% isolated yield, more than 90% selectivity judging from NMR spectra of the crude mixture, Scheme 2). 31P{1H} NMR spectroscopy showed two doublet signals at around 40 ppm with 2JP-P value of 25 Hz, which are very similar to those of the starting complex 6b. 29Si NMR spectroscopy suggested the presence of four different Si atoms, including one SiH2 group. Two of the four Si signals have large JP-Si values (145 and 147 Hz) similar to those of complex 6b (148 and 151 Hz) as well as small JP-Si values (8 and 15 Hz), while the other two Si signals, including the SiH2 signal, have only small JP-Si values (2 and 12 Hz, respectively). 1H NMR spectroscopy showed two signals (both 1H integration) for SiH2 hydrogens at 4.8 and 5.5 ppm, respectively. These data do not agree with a tetrakis(silyl)palladium(IV) complex similar to 3, which should have two SiH2 groups, but rather suggest that the new complex keeps a bis(silyl)(depe)palladium(II) core structure similar to that of 6b with two extra silicon atoms, including one SiH2 group. The structure of 7 was unambiguously determined by (11) For examples of silylpalladium complexes characterized by X-ray diffraction, see: (a) Pan, Y.; Mague, J. T.; Fink, M. J. Organometallics 1992, 11, 3495. (b) Murakami, M.; Yoshida, T.; Ito, Y. Organometallics 1994, 13, 2900. (c) Suginome, M.; Oike, H.; Park, S. S.; Ito, Y. Bull. Chem. Soc. Jpn. 1996, 69, 289. (d) Ozawa, F.; Sugawara, M.; Hasebe, K.; Hayashi, T. Inorg. Chim. Acta 1999, 296, 19. (e) Woo, T. K.; Pioda, G.; Rothlisberger, U.; Togni, A. Organometallics 2000, 19, 2144. (f) Chen, W. Z.; Shimada, S.; Tanaka, M. Science 2002, 295, 308. (g) Boyle, R. C.; Mague, J. T.; Fink, M. J. J. Am. Chem. Soc. 2003, 125, 3228. (h) Lee, Y. J.; Lee, J. D.; Kim, S. J.; Keum, S.; Ko, J. J.; Suh, I. H.; Cheong, M.; Kang, S. O. Organometallics 2004, 23, 203. (i) Tanabe, M.; Mawatari, A.; Osakada, K. Organometallics 2007, 26, 2937. (j) Watanabe, C.; Iwamoto, T.; Kabuto, C.; Kira, M. Angew. Chem., Int. Ed. 2008, 47, 5386. (k) Esposito, O.; Roberts, D. E.; Cloke, F. G. N.; Caddick, S.; Green, J. C.; Hazari, N.; Hitchcock, P. B. Eur. J. Inorg. Chem. 2009, 1844.

Chart 1

a single-crystal X-ray analysis (Figure 1). As suggested by NMR spectroscopy, complex 7 keeps a square-planar bis(silyl)(depe)palladium(II) framework, while it has two new Si-Si bonds, which were produced by a dehydrogenative Si-Si coupling reaction between both Si atoms in 4 and the SiH2 moiety in 6b. The bond distances (Pd-P, Pd-Si, and Si-Si) are all within common ranges.11 The dmpe complex 6a also reacted with 1 equiv of 4 upon heating at 80 °C to give the new complex 8 in 65% isolated yield (Scheme 2). Monitoring the reaction by NMR spectroscopy showed the presence of several uncharacterized species, but the dinuclear palladium complex 9 (Chart 1), which was formed by heating complex 6a alone without 4 at 80 °C in toluene,9 was not observed. 29Si NMR spectroscopy showed the presence of four different Si atoms, including one SiH group but not a SiH2 group, suggesting that complex 8 is not a dmpe analogue of 7 or a complex similar to 3. Two of the four Si signals, including the SiH signal, have large JP-Si values (147 and 155 Hz) typical for square-planar bis(silyl)bis(phosphine)palladium(II) complexes. One of the other two Si signals appeared at a high-field position

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Figure 3. View of a part of complex 8 along the Si3-Si3* axis, showing an eclipsed conformation (symmetry operator for *: -x, y, -z þ 1/2). Scheme 3. Plausible Mechanism for the Formation of 7a

Figure 2. Thermal ellipsoid plots (50% probability level for Pd, P, Si and C atoms) of compound 8: (a) side view; (b) top view. One set of disordered carbon atoms and hydrogen atoms bonded to carbon atoms are omitted for clarity. Selected bond lengths (A˚) and angles (deg) (symmetry operator for *: -x, y, -z þ 1/2): Pd(1)-P(1), 2.3228(8); Pd(1)-P(2), 2.3405(7); Pd(1)-Si(1), 2.3700(7); Pd(1)Si(2), 2.3375(8); Si(2)-Si(3), 2.3486(9); Si(3)-Si(3*), 2.3418(9); Si(3)-Si(4*), 2.3524(8); P(1)-Pd(1)-P(2), 85.35(3); P(1)-Pd(1)Si(1), 98.73(3); P(2)-Pd(1)-Si(2), 96.33(2); Si(1)-Pd(1)-Si(2), 79.68(2); Si(2)-Si(3)-Si(3*), 120.55(3); Si(2)-Si(3)-Si(4*), 121.39(3); Si(3)-Si(3*)-Si(4), 91.93(3).

of -66.68 ppm with a small JP-Si value of 5 Hz, while the remaining signal appeared as a singlet at 3.86 ppm. An X-ray structure analysis unambiguously determined the structure of complex 8 (Figure 2). As suggested by NMR spectroscopy, complex 8 retains square-planar bis(silyl)(dmpe)palladium(II) moieties containing an SiH group. However, it is not a mononuclear complex but a dinuclear complex having two bis(silyl)(dmpe)palladium(II) moieties bridged by a cyclic dimer of 4. Five Si-Si bonds were created during the formation of 8. The high-field 29Si NMR signal at -66.68 ppm can be assigned to the internal silicon atom having three Si-Si bonds. As shown in Figure 3, the 1,5-disilyl-1,2,5,6-tetrasila[3.3.0]bicyclooct-3,7diene moiety, the central part of 8, has an almost completely eclipsed conformation along the Si3-Si3* axis (dihedral angles Si2-Si3-Si3*-Si2* = 1.61(4)° and Si4-Si3*-Si3-C15 = 2.34(7)°).12 The bond distances (Pd-P, Pd-Si, and Si-Si) are all within common ranges. It should be noted that the difference in steric bulkiness of the phosphine ligands (dmpe vs depe) critically affects the reaction products. Also, the formation of 8, having five new Si-Si bonds, in a good yield is remarkable. Previously we have reported related Si-Si bond formation in bis(2-silylphenyl)silane/Pd13 and 4/Ni14 systems, forming complexes (12) For an example of a similar eclipsed structure, see: Herzog, U.; Rheinwald, G. Eur. J. Inorg. Chem. 2001, 3107. (13) Chen, W. Z.; Shimada, S.; Hayashi, T.; Tanaka, M. Chem. Lett. 2001, 1096. (14) Shimada, S.; Rao, M. L. N.; Hayashi, T.; Tanaka, M. Angew. Chem., Int. Ed. 2001, 40, 213.

a

Legend: OA = oxidative addition; RE = reductive elimination.

10 and 11, respectively (Chart 1). Although the mechanisms for the formation of complexes 7 and 8 are not clear, the formation of complex 7 may be explained by a sequence of oxidative addition and reductive elimination steps, as in the case of 10 (Scheme 3).13 The silyl ligand in 11 and the bridging part between the two bis(silyl)palladium moieties in 8 have very similar structures (cyclic dimers of 4). Therefore, the formation of these two complexes probably includes similar mechanisms.

Experimental Section General Procedures. All manipulations of air-sensitive materials were carried out under a nitrogen atmosphere using standard Schlenk techniques or in a glovebox. Toluene, toluene-d8, and THF-d8 were distilled from Na/benzophenone ketyl. All other anhydrous solvents were purchased from Kanto Chemicals or Aldrich. Compounds 4,14 6a,9 and Pd(PEt3)415 were prepared as described in the literature. 1H, 29Si, and 31P NMR spectra were recorded on a JEOL LA500 spectrometer at room temperature. Chemical shifts are given in ppm using external references (tetramethylsilane (0 ppm) for 1H and 29Si (15) Schunn, R. A. Inorg. Chem. 1976, 15, 208.

Note and 85% H3PO4 (0 ppm) for 31P), and coupling constants are reported in hertz. Three different 29Si NMR measurements (INEPT and DEPT (1H decoupled and 1H coupled)) were performed for complexes 7 and 8 to obtain signals for all Si atoms and to clarify the number of H atoms directly bonded to Si atoms. Complex 6b. A mixture of Pd(PEt3)4 (400 mg, 0.691 mmol) and depe (142 mg, 0.689 mmol) in toluene (5 mL) was stirred at room temperature overnight. Volatiles were removed under vacuum to leave complex 5b as a viscous solid. To the solid was added toluene (5 mL) and 4 (115 mg, 0.691 mmol) at 0 °C. During the addition of 4, a small amount of gas evolution was observed. Then the mixture was stirred at room temperature for 12 h under a nitrogen atmosphere. Removing volatiles under vacuum and washing the residual solid with hexane (3  3 mL) gave 6b (240 mg, 73% yield) as a pale brown powder. This powder was used without further purification. 1H NMR (499.1 MHz, toluene-d8): δ 0.69 (dd, 6H, JP-H = 1 Hz, JP-H = 3 Hz), 0.80 (dt, 6H, J = 16, 8 Hz), 0.87 (dt, 6H, J = 16, 8 Hz), 1.06-1.19 (m, 4H), 1.30-1.53 (m, 8H), 5.54 (dd, 2H, JP-H = 9, 10 Hz, JSi-H = 158 Hz), 7.29 (dt, 1H, JH-H = 1, 7 Hz), 7.34 (t, 1H, JH-H = 7 Hz), 7.79 (d, 1H, JH-H = 7 Hz), 8.03 (d, 1H, JH-H = 7 Hz). 31P{1H} NMR (202.0 MHz, toluene-d8): δ 41.0 (d, 2JP-P = 22 Hz, 2JSi-P = 148 Hz), 41.4 (d, 2JP-P = 22 Hz, 2 JSi-P = 151 Hz). 29Si{1H} NMR (99.1 MHz, toluene-d8): δ -19.3 to -17.2 (m, SiH2), 30.3-32.2 (m, SiMe2). Complex 7. To a toluene solution (3 mL) of 6b (200 mg, 0.419 mmol) was added 4 (70 mg, 0.42 mmol) at room temperature, and the mixture was stirred at 90 °C for 2 days. Removing volatiles under vacuum and washing the residual solid with hexane (3  3 mL) gave 7 (190 mg, 71%) as a white powder. 1H NMR (499.1 MHz, THF-d8): δ 0.41 (dt, 3H, J = 15, 7 Hz), 0.47 (s, 3H), 0.49 (s, 3H), 0.50-0.60 (m, 1H), 0.64-0.73 (m, 6H), 0.74-1.61 (m, 20H), 4.84 (d, 1H, J = 3 Hz, JSi-H = 180 Hz), 5.52-5.59 (m, 1H), 7.19-7.27 (m, 2H), 7.29 (t, 1H, JH-H = 7 Hz), 7.35 (t, 1H, JH-H = 7 Hz), 7.61 (d, 1H, JH-H = 7 Hz), 7.76 (d, 1H, JH-H = 7 Hz), 7.93 (d, 1H, JH-H = 7 Hz), 8.15 (d, 1H, JH-H = 7 Hz). 31P{1H} NMR (202.0 MHz, THF-d8): δ 39.4 (d, 2 JP-P = 25 Hz), 41.0 (d, 2JP-P = 25 Hz). 29Si{1H} NMR (99.1 MHz, THF-d8, INEPT, measurement parameters were set to the silicon atoms with two methyl groups): δ -38.8 (dd, 2JP-Si = 15 Hz, 2JP-Si = 147 Hz, Si), -0.7 (d, 3JP-Si = 2 Hz, SiMe2), 31.6 (dd, 2JP-Si = 8 Hz, 2JP-Si = 145 Hz, SiMe2). 29Si{1H} NMR (99.1 MHz, THF-d8, DEPT, measurement parameters were set to the silicon atoms with directly bound hydrogen atoms): δ -48.7 (t, 3JP-Si = 12 Hz, SiH2; the number of hydrogens bound to the silicon atom was determined by the 1H-coupled 29 Si NMR). Anal. Calcd for C26H46P2PdSi4: C, 48.84; H, 7.25. Found: C, 48.74; H, 7.29.

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Complex 8. To a toluene solution (10 mL) of 6a (200 mg, 0.475 mmol) was added 4 (79 mg, 0.47 mmol) at room temperature, and the mixture was stirred at 80 °C for 2 days. Removing volatiles under vacuum and washing the residual solid with hexane (3  3 mL) gave 8 (177 mg, 65%) as a white powder. 1H NMR (499.1 MHz, THF-d8): δ -0.75 (s, 6H), -0.23 (s, 6H), 0.27 (d, 6H, J = 7 Hz), 0.54 (br dd, 6H, J = 1.5, 3 Hz), 0.70 (d, 6H, J = 3 Hz), 1.31 (d, 6H, J=7 Hz), 1.58 (d, 6H, J=7 Hz), 1.60 (d, 6H, J=7 Hz), 1.15-1.86 (m, 8H), 5.84 (dd, 2H, JP-H =11 Hz, JP-H =26 Hz, 1 JSi-H =161 Hz), 6.87-6.93 (m, 4H), 6.98 (dt, 2H, J=1, 7 Hz), 7.04-7.10 (m, 4H), 7.45 (d, 2H, JH-H = 7 Hz), 7.73 (dd, 2H, JH-H = 2, 7 Hz), 8.04 (d, 2H, JH-H = 7 Hz). 31P{1H} NMR (202.0 MHz, THF-d8): δ 13.4 (d, 2JP-P = 19 Hz, 2JSi-P = 147 Hz), 15.5 (d, 2JP-P = 19 Hz, 2JSi-P = 156 Hz). 29Si{1H} NMR (99.1 MHz, THF-d8, INEPT, measurement parameters were set to the silicon atoms with two methyl groups): δ -66.7 (t, 3JP-Si = 5 Hz, Si-SiMe2), 3.9 (s, Si-SiMe2), 28.8 (dd, 2JP-Si=11 Hz, 2JP-Si=155 Hz, PdSiMe2). 29Si{1H} NMR (99.1 MHz, THF-d8, DEPT, measurement parameters were set to the silicon atoms with directly bound hydrogen atoms): δ -14.1 (dd, 2JP-Si=17 Hz, 2JP-Si=147 Hz, SiH; the number of hydrogen bound to the silicon atom was determined by the 1H-coupled 29Si NMR). Anal. Calcd for C44H74P4Pd2Si8: C, 45.38; H, 6.41. Found: C, 45.30; H, 6.57. X-ray Crystallography. Data collection was performed on a Bruker Smart Apex CCD diffractometer (Mo KR radiation, graphite monochromator). Data were corrected for absorption. The structures were solved by the Patterson method. Structure refinement was carried out by full-matrix least squares on F2. Structure solution and refinement were performed using the CrystalStructure software package16 with the SHELX-97 program.17 CCDC 782662 (6b), CCDC 782663 (7), and CCDC 782664 (8) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (Nos. 15350037, 19350035, and 05F05666) from the Ministry of Education, Science, Sports and Culture of Japan. Y.-H.L. thanks the Japan Society for the Promotion of Science (JSPS) for a postdoctoral fellowship. Supporting Information Available: CIF files giving crystallographic data for 6b, 7, and 8. This material is available free of charge via the Internet at http://pubs.acs.org. (16) CrystalStructure version 3.8; Rigaku and Rigaku Americas, 2007. (17) Sheldrick, G. M. SHELXL-97; Universit€at G€ottingen, G€ottingen, Germany, 1997.